From Here at the irs.gov site
7. Supplemental Wages
Supplemental wages are compensation paid in addition to an employee’s regular wages. They include, but are not limited to, bonuses, commissions, overtime pay, payments for accumulated sick leave, severance pay, awards, prizes, back pay, retroactive pay increases, and payments for nondeductible moving expenses. Other payments subject to the supplemental wage rules include taxable fringe benefits and expense allowances paid under a nonaccountable plan. How you withhold on supplemental wages depends on whether the supplemental payment is identified as a separate payment from regular wages. See Regulations section 31.3402(g)-1 for additional guidance for wages paid after January 1, 2007. Also see Revenue Ruling 2008–29, 2008–24 I.R.B. 1149, available at
www.irs.gov/irb/2008-24_IRB/ar08.html.